Since 1985 we have conducted an extensive series of low temperature pumpability studies in two heavy duty diesel engines, the Cummins NTC-400 and the Mercedes-Benz OM 442A. This work has focused primarily on the influence of the Viscosity Index (VI) improver on the low temperature pumpability performance of SAE 15W-40, SAE 10W-30 and SAE 10W-40 engine oils. In addition, we have also addressed the issue of whether or not the current SAE J300 MRV viscosity limit is appropriate for engine oils used in heavy duty diesel engines. Since the MRV viscosity limit is based on a correlation with the pumpability performance of early 1970's model gasoline engines, it would not be surprising if the 30 Pa.s viscosity limit was inappropriate for modern heavy duty diesel engines. This paper focuses on work in the OM 442A engine. The results indicate, that for this engine the current MRV limit of 30 Pa.s is considerably higher then is warranted by actual engine pumpability results. An MRV viscosity on the order of 10 to 15 Pa.s may be more appropriate. It is also clear that the choice of VI Improver plays a significant role in the low temperature pumpability performance of the oils tested.